In a tweet posted on Wednesday night, Warren said Lieberman had “joined” lobbyists for ZTE, which she wrote had “violated serious U.S. sanctions” against Iran and North Korea.

“Should that be legal? No,” she added, arguing in a subsequent tweet for a lifetime ban on lawmakers working as lobbyists.

Lieberman, who ran unsuccessfully for vice president as a Democrat in 2000 and Kasowitz Benson Torres, the firm where he is a lobbyist, did not immediately respond to requests for comment.

But a lobby registration form released on Wednesday showed Lieberman was “retained to undertake an independent assessment of concerns that Members of Congress, Executive branch and American businesses have about any national security vulnerabilities and risks that ZTE products may pose in the US.”

It added that Lieberman would not be lobbying for ZTE, saying he was registered “in the interest of transparency and caution.”

The comments highlighted the politically charged debate in Washington about ZTE and Huawei Technologies Cos Ltd, China’s biggest network equipment companies. The U.S. government says both companies work at the behest of China’s government and sell equipment capable of spying on Americans, charges both companies deny.

In August, U.S. President Donald Trump signed a bill barring the federal government from using ZTE equipment.

Reuters reported last week that Trump was considering an executive order this year to declare a national emergency, banning American companies from using ZTE and Huawei telecommunications gear.

Warren’s tweets came days after the liberal from Massachusetts formed an exploratory committee that would allow her to begin raising money to compete in what is expected to be a crowded Democratic field for the 2020 presidential race. (Reporting by Alexandra Alper; editing by Jonathan Oatis)